The present invention relates to audio amplifiers and in particular to audio amplifiers which can be placed in a mute state or a standby state with a low consumption of electrical energy.
With the introduction of modern audio apparatus with a high-level performance for "high-fidelity" applications, for example, for television sets and for car radios, constructed essentially using integrated circuits, great care has been taken to prevent annoying noise which can be heard by the human ear ("crackles") due to the switching transients of the electronic components, not only during the stages of switching on and switching off, but also during the transitions from a state of normal operation to a mute state or a standby state with low power consumption.
As is known, an operative mute state, usually defined in the technical literature by the term "mute", is generally set from outside using an appropriate control when the user wishes to stop the output of sounds from the loudspeaker connected to the amplifier for only a short period of time.
In order to obtain a "mute" state it is sufficient to prevent the transfer of audio signals to the loudspeaker, while continuing normally to supply the audio amplifier.
The "mute" state can also be automatically set, by appropriate circuits in the audio apparatus, for predetermined time periods during the turn-on or turn-off transients of the audio apparatus so as to eliminate the transfer of noise generated upstream of the audio amplifier connected to the loudspeaker as an output to the loudspeaker.
Even if the "mute" state is set by a circuit contained within the audio amplifier, it cannot, however, be used to eliminate noise generated in the audio amplifier itself which is essentially due to the instability of power output stage during the switching transients.
A low power consumption standby state, usually defined in the technical literature by the term "standby", is that state in which the greatest possible number of components of an audio apparatus and in particular its audio amplifier are placed in a disabled or cut-off condition, while the connections to the power supply are maintained.
Only those components absolutely necessary for resetting the normal functions of the other components when the "standby" state is discontinued remain in operation.
A "standby" state is generally set externally by the user by means of an appropriate control, but may also be set automatically to provide forms of protection, for instance in cases of overloading, until the abnormal operating conditions are removed.
At present the technical solutions used for the initiation of a "standby" state are separate from those used for the initiation of a "mute" stage and separate circuits are used.
As regards the problem of the noise caused by the switching transients, the technical solutions used in modern integrated circuit audio apparatus are separate depending on whether this noise is generated upstream of the audio amplifier or in the audio amplifier itself and in particular in its power output stage.
In the first case, as has been noted, a "mute" state having a limited duration of time can be effectively used only during the time periods of the switching transients in which the noise may be generated.
In the second case, use is made of circuit devices of the type disclosed, for instance, in U.S. Pat. No. 4,371,841 to Eckert et al., which are designed to suppress the switching transients of the components operating along the signal path.